Optical systems used in microscopy or lithography and capable of imaging small structures down to the scale of a few microns or less on the surface of an object are well known. To produce a good image, an optical system must collect enough light reflected from (or transmitted through) the object, separate the details in the image, magnify the image, and render the details visible to the human eye or an optical detector. The numerical aperture (NA) of the optical system is a measure of its ability to gather light and resolve fine object details at a fixed object distance. The optical system's ability to resolve the fine details and produce a good image is hindered by optical aberrations. Most aberrations are caused by artifacts arising from the interaction of light with optical elements (lenses or mirrors). In particular, chromatic aberrations arise from variations in the refractive indices of the optical elements when interacting with the wide range of frequencies found in the spectrum of light, in particular when imaging with visible light. In general, optical aberrations introduce optical defects in the features of an image being observed through an optical system, thereby degrading the optical system's performance.
In an optical microscope, the optical system closest to the object being imaged is referred to as the objective optical system or simply the “objective”; it forms a real, inverted, magnified image of the object. This image is referred to as an intermediate image and resides in the plane of the field stop of the eyepiece. An objective's numerical aperture influences a number of factors including resolving power, working distance, field of view, and the amount of light collected by the objective. The working distance is the space between the front most surface of the objective optical system and the surface the object closest thereto. The working distance determines what is referred to as the “object space”, i.e., the space between the entrance surface of the first lens in the objective and the object plane. The field of view (FOV) is the area of the object, at the working distance, seen through the objective at one time. The resolving power is the ability of an imaging device to separate (see as distinct) points of an object that are located at a small angular distance from each other. NA influences both the resolving power of the objective optical system and the amount of light that it can collect. Generally, the NA represents the range of angles for which light can be delivered to or collected from an object being imaged using a specific objective design. While many factors must be considered when designing objective optical systems, the ultimate goal is to reduce the number of aberrations.
One of the problems arising from imaging with high NA illumination is chromatic aberration. As it is known to persons having ordinary skill in the art, chromatic aberrations arise from variations in the refractive index of material when interacting with different wavelengths of light. Specifically, for example, when white light passes through a lens, the component wavelengths are refracted according to their frequency. In a positive lens, blue light is refracted to the greatest extent followed by the green and red light components. The inability of a lens to bring all of the colors into a common focus results in a slightly different image size and focal point for each predominant wavelength component. This phenomenon is known as axial chromatic aberration. Additional lateral chromatic aberration may occur when white light is focused off-axis or when the white light source is placed even slightly off-axis. Correcting chromatic aberration, in particular, across the visible spectrum including wavelengths ranging from approximately 400 nm (nanometers) to 700 nm is particularly challenging. Normally, a microscope can be thought of as a positive lens. In that sense, the power of the positive lens produces what is known as “undercorrected” axial chromatic aberration. To compensate for it, overcorrected axial chromatic aberration is intentionally generated by adding specially designed optical elements within the microscope's optical system.
Another problem related to imaging with high NA illumination is the possibility of total internal reflection (TIR). Specifically, TIR can occur when light having a large angle of incidence is refracted, especially, in a lens-air interface as discussed infra with respect to FIG. 2. TIR can be prevented by using a coupling fluid whose index of refraction matches as closely as possible that of the front most surface of the objective optical system.
In addition, image field curvature is another imaging aspect to be considered. Specifically, since an image of a sample is generally captured by a sensor, such as CCD (charged coupled device) or CMOS (complementary metal oxide semiconductor) sensor, which has a flat surface, a flat image is required at the plane where the sensor is located. Generally, however, since a microscope can be regarded as a positive lens, the power of the positive lens generates an image having an inward-curving field. The curvature of the resulting image is known as the Petzval curvature. To compensate for inward Petzval curvature, an outward-curving field is intentionally generated by adding specially designed optical elements within the microscope's optical system. Specifically, using a concave mirror has been known to be an effective method for compensating the inward Petzval curvature. It is clear, therefore, that correction of aberrations can a considerable number of lens elements to the objective optical system. This significant increase in the number of optical elements often results in a tight fit and difficult to align, oversized objective system.
In consideration of the above background, previous attempts to increase the numerical aperture and minimize aberrations have been made. International patent application PCT/US2008/078493, published as WO 2009/046137 A1, by Hwang et al., (herein “Hwang”) discloses, for example, an optical imaging system with a catadioptric objective that is claimed to minimize chromatic aberration and optimize the correction of Petzval curvature, so that an image with flat Petzval field may be obtained. The NA in Hwang's optical imaging system, however, is less than 1.
U.S. Pat. No. 7,646,533B2 to Chuang et al. (herein “Chuang”) discloses various embodiments of a small catadioptric objective with ultra-high NA. The objective includes a lens group having at least one focusing lens that forms an intermediate image. The objective further includes at least one field lens located in proximity to the intermediate image, and a catadioptric system positioned to receive the intermediate image from the at least one field lens. The catadioptric system includes at least one Mangin element and can include a meniscus lens element. Although Chuang's system is termed “ultra-high NA”, NA is less than 1.
U.S. Pat. No. 6,600,608 to Shafer et al., (herein “Shafer”) discloses a catadioptric objective that forms two intermediate images. The objective includes two refractive partial objectives and one catadioptric partial objective. The objective includes a first partial objective, a first intermediate a image, a second partial objective, a second intermediate image, and a third partial objective. At least one of the partial objectives is purely refractive. One of the partial objectives is purely refractive and one is purely catoptric. According to Shafer, the catoptric partial objective carries the burden of Petzval sum reduction or field flattening. This relieves the refractive partial objective from the need for beam contractions and expansions by negative and positive lens groups. However, the objective disclosed by Shafer is a projection objective limited to be used in a microlithography projection exposure apparatus that operates primarily in the deep ultra-violet (DUV) region of the electromagnetic spectrum.
Lastly, U.S. Pat. No. 7,884,998 to J. Joseph Armstrong (herein “Armstrong”) discloses a catadioptric microscope objective employing immersion liquid for use in broadband microscopy. The objective receives light energy from a light energy source configured to provide light energy in a wavelength range of approximately 480 to 660 nanometers, employs a Mangin mirror arrangement in conjunction with an immersion liquid to provide a numerical aperture in excess of 1.0 and a field size in excess of 0.05 millimeters. As acknowledged by Armstrong, problems with immersion objectives employed within immersion imaging systems include the inability to resolve the image in the presence of either low wavelength or broad wavelength range light energy, or in the presence of different types of illumination and imaging modes. Further, immersion techniques cannot be universally employed with objectives available in standard microscopes.
Therefore, in spite of the advances attained by the present state of the art, currently available objectives do not allow imaging over a large FOV. To the best of the inventor's knowledge, currently available immersion objectives can at most offer a NA of approximately 1.3, a FOV no higher than approximately 1 millimeter, and wavefront R.M.S. equal to or less than 70 mλ (milli-lambda). No immersion objectives are currently known to the inventor herein that can simultaneously support broad wavelengths in the spectral range of approximately 400 to 700 nm, exhibit large FOV sizes, accept light with high NAs equal to or greater than 1.60, and generate wavefronts with R.M.S. errors equal to or less than 40 mλ.